1. Field of the Invention
This invention relates to a method for manufacturing a hetero-junction bipolar transistor.
2. Related Prior Art
The hetero-junction bipolar transistor (HBT) has an attention for an amplifying device in a high-speed optical communication system due to its large current gain and an excellent high frequency performance. Since the HBT has an emitter with band gap energy greater than that of a base, a minority carrier injection from the base to the emitter is prevented. Therefore, even when a majority carrier concentration in the base is increased, the current gain of the transistor can be kept high. Moreover, to increase the majority carrier in the base can be tied up to a thinner base, thereby shortening a carrier drifting time and enhancing the high frequency performance.
The HBT composed of group III-V semiconductor materials is likely to configure an n-p-n type transistor, because the mobility of electrons is far greater than that of holes in such compound semiconductor. Conventionally, zinc (Zn) is used as a dopant material for the acceptor of the compound semiconductor. However, since the maximum solubility of Zn in compound semiconductor is so limited that the hole concentration is 2xc3x971019 cmxe2x88x923 at a maximum, the base with a high hole concentration can not be obtained. Moreover, since Zn easily diffuses in the compound semiconductor, a doping profile of Zn is likely to widen, thereby increased the equivalent thickness of the base and decreases the performance of the transistor. Further, the diffusion of Zn atom causes the deterioration of the reliability.
Recently, carbon (C) substituting for Zn is going to use as a p-type dopant material for III-V compound semiconductor such as InP, GaAs, InGaP and InGaAsP. The reasons to apply carbon are that the maximum hole concentration exceeds that for the zinc, and carbon hardly diffuses in the semiconductor material. This results in the advantage of the designing the epitaxial layer structure and HBT with a high reliability.
However, it is not always so simple to utilize the advantage of carbon because of its inferior activation efficiency. During the growth of a semiconductor layer doped with carbon, hydrogen atoms or hydrogen radicals are generated due to the decomposition of hydrides compound such as arsine (AsH3) or phosphine (PH3). Carbon atoms can easily combine with hydrogen atoms or hydrogen radicals, make a Cxe2x80x94H bond and is brought within the semiconductor crystal. The carbon with Cxe2x80x94H bond is passivated and is hard to function as an acceptor, which restricts the hole concentration.
An object of the present invention is to provide a method for manufacturing an HBT having a base layer doped with carbon and for enhancing a hole concentration by an activation of the carbon.
The method according to the present invention comprises the steps of: (1) epitaxially growth of a multi-layered semiconductor film having a collector film, a base film containing carbon atoms as dopants, an emitter film and an emitter contact film on a semiconductor substrate, (2) annealing the multi-layered semiconductor film by a predetermined period, and (3) forming the multi-layered semiconductor film to make an collector layer, a base layer, an emitter layer, and an emitter contact layer. The relation between temperatures of the present invention is denoted by T less than TAxe2x89xa6600xc2x0 C., where T is a growth temperature of the emitter film and the emitter contact film and TA is an annealing temperature.
Since the annealing is performed at TA higher than the growth temperature of the emitter film and the emitter contact film, hydrogen captured in the base film can be dissociated therefrom even after the growth of the emitter film and the emitter contact film, thereby enhancing the hole concentration of the base layer. Moreover, since the annealing is performed below 600xc2x0 C., the deterioration of the surface morphology of grown semiconductor layer can be prevented.
The growth temperature T of the emitter film and the emitter contact film is preferably higher than 500xc2x0 C. By setting the growth temperature T to be higher than 500xc2x0 C., hydrogen can dissociate from the base film without deteriorating the crystal quality and the surface morphology of the grown layer.
The annealing period is preferably longer than 5 minutes. By setting the annealing period to be at least 5 minutes, hydrogen captured in the base film can be dissociated therefrom.
The annealing is preferably performed under nitrogen atmosphere. By using the nitrogen for the ambience, it is unnecessary to provide a supplemental pass for supplying an arsenic compound such as Tri-Methyl-Arsine (TMAs), whereby an ordinarily growing apparatus can be applied. Moreover, it is unnecessary to bring the semiconductor substrate out of the reactor chamber.
The present invention may include a further growth of an additional cover layer before annealing and a removal of the additional cover layer after annealing. Inserting and removing the additional cover layer can keep the surface morphology of the multi-layered semiconductor film kept on a good quality.
The HBT of the present invention preferably has an InP for the semiconductor substrate and for the emitter layer, an InGaAs lattice matched to InP for the collector layer, the base layer and the emitter contact layer. Moreover, The collector layer, the emitter layer and the emitter contact layer preferably contain silicon as an n-type dopant.